1. Field of the Invention
The present invention relates to a liquid crystal optical element provided with a liquid crystal which exhibits two or more optically stable states in a non-voltage-application time.
2. Description of the Background
A chiral nematic liquid crystal optical element (hereinbelow, referred to as CL-LCD) has a phase-change type mode. It provides selective reflection in a planar state (hereinbelow, referred to as PL), and provides a scattering state in a focalconic state (hereinbelow, referred to as FC). By applying a predetermined voltage across the electrodes, the liquid crystal can be transformed into PL or FC. For example, when it is transformed from FC to PL, the liquid crystal is once rendered to be a homeotropic state (a state that liquid crystal molecules are aligned vertically to the substrate plane, and hereinbelow, referred to as HO), and then, it is transformed into PL. Then, the liquid crystal is stable in PL or FC even in a non-voltage-application time, and either state can be maintained.
Description will be made as to an optical display state. In FC, a slightly scattering state of incident light is produced, and in PL, a selective reflection of incident light is produced. Further, by adjusting the helical pitch (λAVG) of the liquid crystal layer, an operation mode of “transmittance-scattering” or a color display utilizing colors of selectively reflected light can be effected.
OPTREX Corporation as the assignee of the application has tentative U.S. Ser. No.: 09/813988 (patent application No., filed on Mar. 22, 2001) regarding electrodes for CL-LCD, and a display state of it and so on; U.S. Ser. No. 09/822,344 (filed on Apr. 2, 2001) regarding the relation of a driving method and a display state thereof and so on, and U.S. Ser. No. 09/824,102 (filed on Apr. 3, 2001) regarding a liquid crystal material and a chiral agent. The present application refers to and includes portions in common with and relating to these applications.
Next, description will be made as to a unique problem of CL-LCD originated from a memory state. A first problem is an “image-sticking phenomenon”. This phenomenon is caused under the following conditions, for example. The state of CL-LCD is made to be a memory state in FC or PL, and it is left for a predetermined time in a non-voltage-application state. Then, even when a voltage corresponding to a new image is applied so as to change the display, the previous image remains.
In the following, such image-sticking phenomenon will be explained by taking a dot matrix type CL-LCD having 160 row electrodes and 160 column electrodes as an example. In this case, it is assumed that “an alignment layer” in contact with the liquid crystal layer at an inner side of the substrates is made of the same material as the one used for a TN or STN liquid crystal display element. Generally, since a surface of an aligning layer in the TN or STN liquid crystal display element is subjected to an aligning treatment by rubbing, a material having a high surface hardness (for example, 3H-6H in a pencil hardness test method) is used. In CL-LCD too for example, the same technique is used. As a concrete example on an aligning layer in contact with the liquid crystal layer, there is known a case of providing polyimide or a case without using any aligning layer (U.S. Pat. No. 5,453,863).
Then, a background portion of a display area of dot matrix type is made to be FC, and portions corresponding to characters, figures and so on are made to be PL. An example of displaying a character of “A” is shown in FIG. 4(a). After such predetermined image has been provided, the application of a voltage is stopped, and the display panel is left for a long time in an incubator of 60° C.
Then, a voltage which makes the liquid crystal in the whole display area to be HO is applied. When the liquid crystal in the whole area becomes HO, the display in the whole area is extinguished. When a voltage is applied subsequently so that the liquid crystal in the whole area becomes FC, the whole area does not provide a uniform color but the character of “A” which has been previously displayed remains slightly as shown in FIG. 4(b). Further, even in a case that the liquid crystal of the whole area is made to be PL via HO after the panel has been left for a long time, the display of “A” remains.
On the contrary, the image-sticking phenomenon takes place even in the following case. Namely, the liquid crystal in the background portion is made to be PL, and the liquid crystal in the portions corresponding to characters and so on is made to be FC. The display panel in such states is left for a long time, and then, a display in the whole area is extinguished by applying a voltage by which the state of the liquid crystal in the whole area of display becomes HO (the liquid crystal itself is in a transparent state). Subsequently, the whole display area is changed to PL or FC. In particular, the image-sticking phenomenon is apt to be observed in a case that a display state wherein the liquid crystal corresponding to the background portion is FC and the liquid crystal corresponding to characters and so on is PL is left for a long time, and then, the liquid crystal in the whole display area is made to be FC.
Further, image-sticking takes place as well in a case of CL-LCD having segment type electrodes. For instance, the image-sticking occurs when a display state wherein a display portion is PL and a background portion is FC is maintained for a long time, and then, such display is changed to another display.
In FIG. 5(a), all 7 segments are ON to display a character of “8”. A color display in combination of black in a background portion in FC and a selective reflection in a visible range in a segment portion in PL, is provided. In FIG. 5(b), when a character of “5” is newly displayed, upper right and lower left segment portions are lit weakly.
However, when a display is performed in combination of PL and FC in CL-LCD, a PL portion is generally light and the highest visibility is obtainable. Accordingly, in a case of the segment type display in FIG. 5, a possibility of causing erroneous recognition is relatively lower than the case of the dot matrix type in FIG. 4.
The image-sticking phenomenon remains slightly even after a voltage for providing HO has continuously been applied and a state that the entire display area is erased has been continued for several hours. Further, in some cases, a display state having the image-sticking phenomenon (hereinbelow, also referred to as image-sticking) cannot be completely erased even after an isotropic state which is caused by heating the liquid crystal has been continued for several hours.
Accordingly, it is an object of the present invention to provide CL-LCD capable of preventing the occurrence of image-sticking even though the liquid crystal is left for a long time while the state of the liquid crystal is constant.
Next, a second problem is the brightness in display of CL-LCD. CL-LCD provides brighter display than TN-LCD or STN-LCD since CL-LCD can be basically used without using a polarizing plate. However, without carrying out alignment control, it is difficult to obtain sufficient brightness that is supposed to be exhibited. From this viewpoint, CL-LCD basically provides insufficient brightness in the “display” to be provided for a user in some cases.
This is because the brightness in the display is directly affected by the dispersion in the direction of helical axes in the entire liquid crystal layer in CL-LCD. Generally speaking, wider dispersions make a viewing angle wider and the display darker. Conversely, narrower dispersions make the viewing angle smaller and the display brighter. In principle, there is a contradiction between the size in the viewing angle and the brightness in the display in CL-LCD.
It is another object of the present invention to ensure a certain view and a certain degree of viewing angle but also attain bright display simultaneously in addition to prevention against the “image-sticking” stated earlier.
The inventors have conducted further research and development on the structure in a cell of CL-LCD and have found that CL-LCD capable of obtaining remarkably bright display having a high contrast ratio can be provided by collecting light in an observing direction by a user with a certain degree of viewing angle ensured.
Further, the present invention is to provide a liquid crystal optical element having excellent function without changing largely the conventional manufacturing method, and a test method for the liquid crystal optical element.